Techniques for Radio Link Problem and Recovery Detection in a Wireless Communication System

ABSTRACT

A technique for radio link detection in a wireless communication system includes estimating a first error rate of an indicator channel. In this case, the indicator channel includes an indication of a number of symbols in a control channel. A second error rate of the control channel is also estimated. The first and second error rates are then combined to provide a performance metric. Based on the performance metric, a determination is made as to whether a radio link problem exists.

BACKGROUND

1. Field

This disclosure relates generally to a wireless communication systemand, more specifically, to techniques for radio link problem andrecovery detection in a wireless communication system.

2. Related Art

As is well known, a wireless channel provides an arbitrary timedispersion, attenuation, and phase shift in a transmitted signal. Whilethe implementation of orthogonal frequency division multiplexing (OFDM)with a cyclic prefix in a wireless communication system mitigates theeffect of time dispersion caused by a wireless channel, in order toapply linear modulation schemes it is also usually necessary to removeamplitude and phase shift caused by the wireless channel. Channelestimation is typically implemented in a wireless communication systemto provide an estimate (from available pilot information) of anamplitude and phase shift caused by a wireless channel. Equalization maythen be employed in the wireless communication system to remove theeffect of the wireless channel and facilitate subsequent symboldemodulation. Channel tracking is also usually employed to periodicallyupdate an initial channel estimation. For example, channel tracking maybe employed to facilitate periodic frequency-domain and time-domainchannel correlation and periodic updating of channel signal-to-noiseratio (SNR), channel delay spread, and channel Doppler effect.

Known approaches for detecting a radio link problem (RLP) and a radiolink recovery (RLR) in wireless communication systems suffer fromsignificant shortcomings that affect the accuracy and/or testability ofa detection approach. Unfortunately, without a good RLP/RLR metric thatis both accurate, easily implementable, and testable, wirelesscommunication system performance inevitably degrades. For example, in athird-generation partnership project long-term evolution (3GPP LTE)compliant wireless communication system, user equipment (UE) must beable to accurately detect a radio link problem (RLP) and a radio linkrecovery (RLR) to prevent performance degradation of the system.

A known first approach for determining an RLP and an RLR at a UE in anLTE compliant wireless communication system has proposed using aphysical control format indicator channel (PCFICH) and a pseudo-errorrate that is based on received symbols. Unfortunately, the firstapproach may be inaccurate as there is no clear indication of whetherthe decoded message is correct due to the absence of an error detectioncode (e.g., a cyclic redundancy check (CRC)) and the small number ofsubcarriers associated with the PCFICH (e.g., 16 subcarriers in an LTEsystem). A known second approach for determining an RLP and an RLR at aUE in an LTE compliant wireless communication system has proposed usinga combination of an actual PCFICH and an actual physical downlinkcontrol channel (PDCCH) to detect radio link failure. Unfortunately, aUE may not always get a PDCCH grant and, thus, logging a CRC error rateis usually inaccurate as the UE cannot distinguish between a truedecoding error and the absence of a PDCCH grant. In addition, the secondapproach also uses the actual PCFICH which does not have an associatedCRC.

A known third approach for determining an RLP and an RLR at a UE in awireless communication system has proposed using a hypothetical PDCCHtransmission to map to an estimate of a block error rate (BLER) as themetric for RLF detection. While overcoming most of the shortcomings inthe above-referenced approaches, the third approach still neglects thefact that a successful PCFICH decoding is necessary before a PDCCH canbe decoded correctly. As such, the third approach may provide an overlyoptimistic result, which could be problematic in environments with lowsignal-to-interference and noise ratios (SINRs) where radio link failure(RLF) is expected to occur. Furthermore, the third approach is notparticularly feasible from a conformance testing point of view as aPDCCH error rate is not observable without PCFICH errors. As such, a UEmay experience behavior consistency problems when the third approach isemployed.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and is notlimited by the accompanying figures, in which like references indicatesimilar elements. Elements in the figures are illustrated for simplicityand clarity and have not necessarily been drawn to scale.

FIG. 1 is a diagram of a relevant portion of an example downlink (DL)frame transmitted from a serving base station (BS) in a long-termevolution (LTE) compliant wireless communication system.

FIG. 2 is an example diagram of a relevant portion of a frequencyspectrum that depicts a channel (e.g., a physical control formatindicator channel (PCFICH) or a physical downlink control channel(PDCCH)) that is to be estimated based on reference signals (RSs)located on opposite sides of the channel, according to one embodiment ofthe present invention.

FIG. 3 is a flowchart of an example process for performing radio linkproblem (RLP), radio link failure (RLF), and radio link recovery (RLR)detection, according to one aspect of the present invention.

FIG. 4 is a flowchart of an example process for performing RLP, RLF, andRLR detection, according to another aspect of the present invention.

FIG. 5 is a block diagram of an example wireless communication systemthat includes wireless communication devices that may perform RLP, RLF,and RLR detection according to various embodiments of the presentinvention.

DETAILED DESCRIPTION

In the following detailed description of exemplary embodiments of theinvention, specific exemplary embodiments in which the invention may bepracticed are described in sufficient detail to enable those skilled inthe art to practice the invention, and it is to be understood that otherembodiments may be utilized and that logical, architectural,programmatic, mechanical, electrical and other changes may be madewithout departing from the spirit or scope of the present invention. Thefollowing detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is defined onlyby the appended claims and their equivalents. In particular, althoughthe preferred embodiment is described below in conjunction with asubscriber station (SS), such as a cellular handset, it will beappreciated that the present invention is not so limited and maypotentially be embodied in various wireless communication devices.

As used herein, the term “channel” includes one or more subcarriers,which may be adjacent or distributed across a frequency band. Moreover,the term “channel” may include an entire system bandwidth or a portionof the entire system bandwidth. As used herein, the term “referencesignal” is synonymous with the term “pilot signal.” As is also usedherein, the term “subscriber station” is synonymous with the term “userequipment,” which includes a wireless communication device that may (ormay not) be mobile. In general, a reference signal (RS), when receivedat a subscriber station (SS), is utilized by the SS to perform channelestimation. The disclosed techniques are contemplated to be applicableto systems that employ a wide variety of signaling techniques, e.g.,orthogonal frequency division multiplex (OFDM) signaling andsingle-carrier frequency division multiple access (SC-FDMA) signaling.As used herein, the term “coupled” includes a direct electricalconnection between blocks or components and an indirect electricalconnection between blocks or components achieved using one or moreintervening blocks or components.

In general, accurate channel estimation is desirable to achieveacceptable performance for SSs in a wireless communication system (e.g.,a long-term evolution (LTE) wireless communication system) as downlink(DL) performance is determined by the accuracy of channel estimation. Inan LTE compliant system, RSs are distributed in a subframe and, as such,interpolation may be used to perform channel estimation for an entiretime-frequency grid of an OFDM signal. In the case of an LTE compliantsystem having a 1.4 MHz system bandwidth, only twelve downlink referencesignal (DLRS) subcarriers are currently allocated (in a first symbol ofeach subframe) for channel estimation. It should be appreciated thatwhile the discussion herein is directed to an LTE compliant system, thetechniques disclosed herein are broadly applicable to improving radiolink failure and recovery detection in any wireless communication systemthat employs an indicator channel that includes an indication of anumber of symbols in an associated control channel.

In a 3GPP-LTE wireless communication system, it is desirable (for propersystem operation) for an SS to accurately detect a radio link problem(RLP) and a radio link recovery (RLR). The detection of an RLP may leadto a radio link failure (RLF) detection, i.e., an RLP for a sustainedperiod of time, at which point an SS can shut off an associatedtransmitter independent from network commands and, in this manner,prevent the SS from causing excessive interference on an uplink (UL). Onthe other hand, an RLR detection results in the SS switching theassociated transmitter back on to facilitate a UL connection. Accordingto various aspects of the present disclosure, techniques are disclosedthat accurately and efficiently detect an RLP and an RLR.

According to various embodiments of the present disclosure, ahypothetical physical control format indicator channel (PCFICH)transmission and a hypothetical physical downlink control channel(PDCCH) transmission are combined (e.g., added or scaled and then added)to provide a quality metric for RLP, RLF, and RLR detection. The qualitymetric may take various, forms, e.g., a signal-to-interference and noiseratio (SINR) or a signal-to-noise ratio (SNR). The quality metric forthe hypothetical channels may be estimated by performing interpolationbased on reference signals (pilot signals) that are, for example,located on opposite sides of the hypothetical channels.

In one embodiment, a two-step exponential effective SINR mapping (EESM)is employed to map an estimated SINR for resource elements for which thePCFICH would be transmitted to an error rate and map an estimated SINRfor resource elements for which the PDCCH would be transmitted to anerror rate. The error rates for the hypothetical PCFICH and PDDCH maythen be combined (e.g., added or scaled and then added) to estimate acombined (PCFICH and PDCCH) error rate. The estimated combined errorrate may be averaged over time (e.g., 200 milliseconds for RLP and 100milliseconds for RLR) after which the averaged combined error rate maybe compared against thresholds (e.g., Qin and Qout, respectively) todetermine RLP and RLR, respectively. That is, if the average combinederror rate is greater than Qout, an RLP is detected. Similarly, if theaverage combined error rate is less than Qin, an RLR is detected. In analternative embodiment, quality metrics for the PCFICH and the PDDCH maybe combined and then mapped to an error rate. In the alternativeembodiment, the quality metrics may be scaled prior to being combined.In yet another alternative embodiment, a combined quality metric isderived for the PCFICH and the PDDCH. The combined quality metric isthen mapped to an error rate.

It should be appreciated that a wide variety of transmission formats forboth PCFICH and PDCCH may be employed for determining a quality metric.A control format indicator (CFI) transmitted on PCFICH may be set to,for example, three (CFI=3), which implies three orthogonal frequencydivision multiplexing (OFDM) symbols for control if a bandwidth (BW)configuration has greater than six resource blocks (RBs) and four OFDMsymbols otherwise. As one example, a ‘format 0’ PDCCH transmission ineight control channel elements (CCEs) for RLP detection and a ‘format1C’ PDCCH transmission in four CCES for RLR detection may be assumed.

In general, when an SS cannot decode even the most reliable controlmessage, an RLP should be declared. If an SS can successfully decode abroadcast message, which is typically transmitted in ‘format 1C’ for anLTE compliant system, then the SS can be considered to be insynchronization with a serving base station (BS). Alternatively, a sameformat, e.g., ‘format 0’ may be assumed for both RLP and RLR detectionwhile performing the estimates. For ease of understanding herein, a sameformat is assumed for both RLP and RLR. In a typical implementation, atable of constants that determine a specific mapping function fromsubcarrier SINRs into an effective SINR (that can be used to estimate anerror rate (e.g., a block error rate (BLER)) for both PCFICH and PDCCH)is developed off-line. For example, using an EESM approach a beta (β)value for each transmission configuration (e.g., 1×2, 2×2 spacefrequency block coding (SFBC), and 4×2 SFBC) and for each format for aPCFICH and a PDCCH may be computed off-line and stored in a look-uptable.

For each subframe ‘n’ in a connected mode, the SS estimates a receivedSINR (γi) for each subcarrier corresponding to a hypothetical PCFICHtransmission and computes an effective SINR (γeff), e.g., using an EESMapproach, as follows:

$\gamma_{eff} = {{- \beta}\; {\ln \left( {\frac{1}{N}{\sum\limits_{i = 1}^{N}\; ^{- \frac{\gamma_{i}}{\beta}}}} \right)}}$

where N is the number of subcarriers for a channel and beta (β) is aconstant that depends on a channel format and a system configuration.The effective SINR (γeff) may then be used to determine the PCFICH BLERin additive white Gaussian noise (AWGN), which is the BLER estimate forthe PCFICH transmission (referred to herein as PCFICH_BLER). In atypical implementation, the above steps are then performed for the PDCCHto estimate the PDCCH BLER (referred to herein as PDCCH_BLER). A runningaverage of both the PCFICH_BLER and PDCCH_BLER may then be performed.For example, one can either use a true average, an exponential average,or a weighted average as set forth below:

PCFICH_BLER(n)=αPCFICH_BLER(n−1)+(1−α)PCFICH_BLER

PDCCH_BLER(n)=αPCFICH_BLER(n−1)+(1−α)PDCCH_BLER

where 0<α<1 is an averaging constant related to the averaging windowlength (e.g., 200 subframes, which may correspond to 200 milliseconds inan LTE compliant system).

After an initial period of the window length and at an end of eachreporting period (e.g., 10 frames, which may correspond to 100milliseconds in an LTE compliant system) thereafter, a combined(PCFICH/PDCCH) error rate may be estimated, for example, as follows:

Combined_BLER(n)=1−(1−PCFICH_BLER(n))*(1−PDCCH_BLER(n))

A routine for reporting an RLP may be implemented, for example, usingthe following code:

while Combined_BLER(n) > Qout Report radio link problem to higher layersif Combined_BLER(n) < Qin Radio link recovery detected break fromreporting RLP end end

Since a true successful PDCCH decoding requires a likewise successfulPCFICH decoding, the techniques described herein provide a more accurateperformance metric for RLF and RLR detection than known approaches,especially in environments with low SINRs where RLP is expected tooccur. Furthermore, the disclosed techniques are readily testable asPDCCH can only be tested when PCFICH is also present and the presence ofa PDCCH CRC can accurately determine a true error rate, which can beused as the benchmark for the accuracy of the RLP or RLR detected by theSS in conformance testing.

According to one embodiment of the present disclosure, a technique forradio link detection in a wireless communication system includesestimating a first error rate of an indicator channel. In this case, theindicator channel includes an indication of a number of symbols in acontrol channel. A second error rate of the control channel is alsoestimated. The first and second error rates are then combined to providea performance metric. Based on the performance metric, a determinationis made as to whether a radio link problem exists.

According to another embodiment of the present disclosure, a wirelesscommunication device includes a receiver and a processor coupled to thereceiver. The processor is configured to estimate a first error rate ofan indicator channel. In this case, the indicator channel includes anindication of a number of symbols in a control channel. The processor isalso configured to estimate a second error rate of the control channel.The processor is further configured to combine the first and seconderror rates to provide a performance metric. The processor is alsoconfigured to determine whether a radio link problem exists based on theperformance metric.

According to another aspect of the present disclosure, a technique forradio link detection in a wireless communication system includesestimating a combined effective signal-to-interference and noise ratiofor a control channel and an indicator channel, which includes anindication of a number of symbols in the control channel. The combinedeffective signal-to-interference and noise ratio is then mapped to ablock error rate. Based on the block error rate, a determination is madeas to whether a radio link problem, a radio link failure, or a radiolink recovery exists.

With reference to FIG. 1, a relevant portion of an example downlinkframe 100, which is transmitted from a serving base station (BS) in anLTE compliant system, is illustrated. As is shown, the frame 100 (whichis 10 milliseconds in length) includes ten subframes (each of which are1 millisecond in length). Each of the subframes begins with a symbolthat includes, among other items, one or more reference signals (RSs), aphysical control format indicator channel (PCFICH) and one or morephysical downlink control channels (PDCCH) (labeled‘DLRS/PCFICH/PDCCH’). In the illustrated example, a DL subframe includestwo slots, each of which include seven long blocks (LBs) which encode asymbol. It should be appreciated that the techniques disclosed hereinare broadly applicable to UL subframes that employ more or less than theillustrated number of LBs. With reference to Slot0, a 1^(st) secondarysynchronization channel (SSCH) is assigned to LB 6 and a primarysynchronization channel (PSCH) is assigned to LB 7. With reference toSlot11, a 2^(nd) SSCH is assigned to LB 6 and the PSCH is also assignedto LB 7. With reference to Slot1, a primary broadcast channel (PBCH) isassigned to LB 1 (labeled ‘DLRS/PBCH’) and LBs 2-4.

With reference to FIG. 2, an example diagram of a relevant portion of afrequency spectrum 200 that depicts a channel 206 (e.g., a physicalcontrol format indicator channel (PCFICH) or a physical downlink controlchannel (PDCCH)) that is to be estimated based on reference signals(RSs) 202 and 204, which are located on opposite sides of the channel,is illustrated. As mentioned above, a quality metric for the channel 206may be derived through interpolation of quality metrics associated withthe RSs 202 and 204. Alternatively, a quality metric for the channel 206may derived in another manner.

Turning to FIG. 3, an example process 300 for performing radio linkproblem (RLP), radio link failure (RLF), and radio link recovery (RLR)detection, according to one aspect of the present invention, isdepicted. The process 300 is initiated at block 302, at which pointcontrol transfers to block 304. In block 304, a control unit (e.g., aprocessor or an application specific integrated circuit (ASIC)) of an SSestimates a first error rate (e.g., a block error rate (BLER)) for anindicator channel, which includes an indication of a number of symbolsin a control channel. As noted above, the first error rate may beestimated based on various quality metrics (e.g., SNR or SINR)associated with reference signals (RSs) located on opposite sides of theindicator channel. For example, the control unit may estimate aneffective SINR for the indicator channel (based on interpolation of theSINRs of respective RSs located on opposite sides of the indicatorchannel) and map the estimated SINR for the indicator channel to thefirst error rate. Next, in block 306, the control unit of the SSestimates a second error rate (e.g., a BLER) for the control channel. Asmentioned above, the second error rate may also be based oninterpolation of various quality metrics associated with RSs located onopposite sides of the control channel. For example, the control unit mayestimate an effective SINR for the control channel using interpolationof the SINRs associated with respective RSs located on opposite sides ofthe control channel and map the estimated SINR for the control channelto the second error rate.

Then, in block 308, the first and second error rates (which may beBLERs) are combined (e.g., added together or scaled and then addedtogether) to provide a performance metric (e.g., a combined BLER). Next,in decision block 310, the control unit determines whether an RLP isindicated by the performance metric (e.g., whether the combined BLER isabove a first threshold). If an RLP is indicated in block 310, controltransfers to block 316, where the RLP is reported, e.g., to a higherlayer. For example, when multiple RLPs have been reported, the higherlayer may initiate power-down of a transmitter of an SS such that the SSdoes not cause excessive interference on a UL. Then, in decision block318, the processor determines whether the RLP has been persistent for afirst time period (e.g., whether the combined BLER has exceeded thefirst threshold for the first time period).

If an RLP is not persistent for the first time period, control transfersfrom block 318 to block 322 where the process 300 terminates and controlreturns to a calling routine. If an RLP is persistent for the first timeperiod, control transfers to block 320 where an RLF is reported. Fromblock 320 control transfers to block 322. If an RLP is not indicated inblock 310, control transfers to decision block 312, where the controlunit determines if an RLR is indicated by the performance metric (e.g.,whether the combined BLER is below a second threshold). If an RLR isindicated in block 312, control transfers to block 314, where the RLR isreported. Following block 314, control transfers to block 322. If an RLRis not indicated in block 312, control transfers to block 322.

With reference to FIG. 4, a process 400 for radio link detection in awireless communication system, according to another embodiment of thepresent disclosure, is illustrated. In block 402 the process 400 isinitiated at which point control transfers to block 404. In block 404, acombined quality metric (e.g., an SNR or an SINR) for a control channeland an indicator channel, which includes an indication of a number ofsymbols in the control channel, is estimated. The combined qualitymetric may, for example, be based on interpolation of quality metricsassociated with respective RSs located on opposite sides of thechannels. The quality metrics for the control and indicator channelsmay, for example, be combined by addition or may be scaled and thenadded together. Next, in block 406, the quality metric is mapped to anerror rate (e.g., a BLER). Then, in block 408, based on the error rate,a determination is made (based on whether the error rate is above afirst threshold, above the first threshold for a first time period, orbelow a second threshold) as to whether a radio link problem, a radiolink failure, or a radio link recovery exists. From block 408 controltransfers to block 410 where the process 400 terminates and controlreturns to a calling routine.

With reference to FIG. 5, an example wireless communication system 500is depicted that includes a plurality of subscriber stations or wirelesscommunication devices 502, e.g., hand-held computers, personal digitalassistants (PDAs), cellular telephones, etc., that may perform radiolink problem (RLP), radio link failure (RLF), and radio link recovery(RLR) detection according to one or more embodiments of the presentdisclosure. In general, the devices 502 include a processor 508 (e.g., adigital signal processor (DSP) or an ASIC), a transceiver (including areceiver and a transmitter) 506, and one or more input/output devices504 (e.g., a camera, a keypad, display, etc.), among other componentsnot shown in FIG. 5. As is noted above, according to various embodimentsof the present disclosure, techniques are disclosed that generallyimprove RLP, RLF, and RLR detection. The devices 502 communicate with abase station controller (BSC) 512 of a base station subsystem (BSS) 510,via one or more base transceiver stations (BTS) 514, to receive ortransmit voice and/or data and to receive control signals. In general,the BSC 512 may also be configured to choose a modulation and codingscheme (MCS) for each of the devices 502, based on channel conditions.

The BSC 512 is also in communication with a packet control unit (PCU)516, which is in communication with a serving general packet radioservice (GPRS) support node (SGSN) 522. The SGSN 522 is in communicationwith a gateway GPRS support node (GGSN) 524, both of which are includedwithin a GPRS core network 520. The GGSN 524 provides access tocomputer(s) 526 coupled to Internet/intranet 528. In this manner, thedevices 502 may receive data from and/or transmit data to computerscoupled to the Internet/intranet 528. For example, when the devices 502include a camera, images may be transferred to a computer 526 coupled tothe Internet/intranet 528 or to another one of the devices 502. The BSC512 is also in communication with a mobile switching center/visitorlocation register (MSC/VLR) 534, which is in communication with a homelocation register (HLR), an authentication center (AUC), and anequipment identity register (EIR) 532. In a typical implementation, theMSC/VLR 534 and the HLR, AUC, and EIR 532 are located within a networkand switching subsystem (NSS) 530, which performs various functions forthe system 500. The SGSN 522 may communicate directly with the HLR, AUC,and EIR 532. As is also shown, the MSC/VLR 534 is in communication witha public switched telephone network (PSTN) 542, which facilitatescommunication between wireless devices 502 and land telephone(s) 540.

As used herein, a software system can include one or more objects,agents, threads, subroutines, separate software applications, two ormore lines of code or other suitable software structures operating inone or more separate software applications, on one or more differentprocessors, or other suitable software architectures.

As will be appreciated, the processes in preferred embodiments of thepresent invention may be implemented using any combination of computerprogramming software, firmware or hardware. As a preparatory step topracticing the invention in software, the computer programming code(whether software or firmware) according to a preferred embodiment willtypically be stored in one or more machine readable storage mediums suchas fixed (hard) drives, diskettes, optical disks, magnetic tape,semiconductor memories such as read-only memories (ROMs), programmableROMs (PROMs), etc., thereby making an article of manufacture inaccordance with the invention. The article of manufacture containing thecomputer programming code is used by either executing the code directlyfrom the storage device, by copying the code from the storage deviceinto another storage device such as a hard disk, random access memory(RAM), etc., or by transmitting the code for remote execution. Themethod form of the invention may be practiced by combining one or moremachine-readable storage devices containing the code according to thepresent invention with appropriate standard computer hardware to executethe code contained therein. An apparatus for practicing the inventioncould be one or more computers and storage systems containing or havingnetwork access to computer program(s) coded in accordance with theinvention.

Although the invention is described herein with reference to specificembodiments, various modifications and changes can be made withoutdeparting from the scope of the present invention as set forth in theclaims below. For example, many of the techniques disclosed herein arebroadly applicable to a wide variety of wireless communication systems.Accordingly, the specification and figures are to be regarded in anillustrative rather than a restrictive sense, and all such modificationsare intended to be included with the scope of the present invention. Anybenefits, advantages, or solution to problems that are described hereinwith regard to specific embodiments are not intended to be construed asa critical, required, or essential feature or element of any or all theclaims.

Unless stated otherwise, terms such as “first” and “second” are used toarbitrarily distinguish between the elements such terms describe. Thus,these terms are not necessarily intended to indicate temporal or otherprioritization of such elements.

1-20. (canceled)
 21. A method, comprising: at a subscriber station,estimating a first error rate for an indicator channel; estimating asecond error rate for a control channel; comparing one of the firsterror rate and the second error rate to a first threshold to determine aradio link problem; and comparing one of the first error rate and thesecond error rate to a second threshold to determine a radio linkrecovery.
 22. The method of claim 21, further comprising: combining thefirst and second error rates to generate a performance metric, whereinthe comparing is based on the performance metric.
 23. The method ofclaim 21, further comprising: providing an indication when the comparingindicates a radio link problem.
 24. The method of claim 21, furthercomprising: providing an indication when the comparing indicates a radiolink recovery.
 25. The method of claim 21, wherein the indicator channeloccupies a first frequency band and estimating of the first error rateis based on a first reference signal occupying a first frequencyadjacent to the first frequency band and a second reference signaloccupying a second frequency adjacent to the first frequency band,wherein the first ad second frequencies are on opposite sides of thefirst frequency bands.
 26. The method of claim 25, wherein the firsterror rate is based on a first quality metric of the first referencesignal and a second quality metric of the second reference signal. 27.The method of claim 26, wherein the first error rate is based oninterpolating the first quality metric and the second quality metric.28. The method of claim 21, wherein the indicator channel includes anindication of a number of symbols in the control channel.
 29. The methodof claim 23, further comprising: determining when the radio link problemexists for a period of time; indicating a radio link failure when theradio link problem exists for the period of time.
 30. The method ofclaim 21, wherein the method is performed for a plurality of indicatorchannels and a corresponding plurality of control channels and when apredetermined number of radio link problems exist, the method furthercomprises: powering down a transmitter of the subscriber station.
 31. Awireless communication device, comprising: a processor configured to:estimate a first error rate for an indicator channel; estimate a seconderror rate for a control channel; compare one of the first error rateand the second error rate to a first threshold to determine a radio linkproblem; and compare one of the first error rate and the second errorrate to a second threshold to determine a radio link recovery.
 32. Thewireless communication device of claim 31, wherein the processor isfurther configured to: combine the first and second error rates togenerate a performance metric, wherein the comparing is based on theperformance metric.
 33. The wireless communication device of claim 31,wherein the processor is further configured to: provide an indicationwhen the comparing indicates a radio link problem.
 34. The wirelesscommunication device of claim 31, wherein the processor is furtherconfigured to: provide an indication when the comparing indicates aradio link recovery.
 35. The wireless communication device of claim 31,wherein the indicator channel occupies a first frequency band and theprocessor estimates the first error rate based on a first referencesignal occupying a first frequency adjacent to the first frequency bandand a second reference signal occupying a second frequency adjacent tothe first frequency band, wherein the first and second frequencies areon opposite sides of the first frequency bands.
 36. The wirelesscommunication device of claim 35, wherein the processor estimates thefirst error rate based on a first quality metric of the first referencesignal and a second quality metric of the second reference signal. 37.The wireless communication device of claim 36, wherein the processorestimates the first error rate by interpolating the first quality metricand the second quality metric.
 38. The wireless communication device ofclaim 31, wherein the indicator channel includes an indication of anumber of symbols in the control channel.
 39. The wireless communicationdevice of claim 33, wherein the processor is further configured to:determine when the radio link problem exists for a period of time;indicate a radio link failure when the radio link problem exists for theperiod of time.
 40. The wireless communication device of claim 31,wherein the processor estimates and compares for a plurality ofindicator channels and a corresponding plurality of control channels andwhen a predetermined number of radio link problems exist, the processoris further configured to: power down a transmitter of the wirelesscommunication device.